Capacitor discharge type ignition system for internal combustion engines

ABSTRACT

A capacitor discharge type ignition system for internal combustion engines comprising an ignition coil whose primary winding is connected at one end thereof to one terminal of a discharge capacitor, has an intermediate tap connected through a switching element to the other terminal of the capacitor, and is connected at the other end thereof to the aforesaid other terminal of the capacitor through a diode inserted in the polarity opposite to that of the switching element or a series circuit consisting of a diode of the above-mentioned polarity and a choke coil.

United States Patent Oishi [54] CAPACITOR DISCHARGE TYPE IGNITION SYSTEMFOR INTERNAL COMBUSTION ENGINES [72] Inventor: Kazuo Oishi, Kariya,Japan [73] Assignee: Nippondenso Kabushiki Kaisha, Kariyashi, Japan [22]Filed: Feb. 27, 1970 [21] App1.No.: 14,969

[30] Foreign Application Priority Data Apr. 11, 1969 Japan ..44/28535Apr. 12, 1969 Japan ..44/28405 [52] US. Cl ..123/l48 E, 315/209 [51]Int. Cl ..F02p 3/06 [58] Field of Search l23/ l48 E; 315/209 [56] 7References Cited UNITED STATES PATENTS 3,329,867 7/1967 Steams 123/148 E51 May 30, 1972 3,382,407 5/1968 Dotto ..l23/148 E 3,291,110 12/1966Peters ....123/l48 E 3,234,430 2/1966 lssler et a1.. ..3l5/209 3,322,1065/1967 Earp ..l23/148 E 3,331,362 7/1967 Mitchell 123/148 E PrimaryExaminer-Laurence M. Goodridge Assistant ExaminerCort FlintAttorney-Cushman, Darby & Cushman [57] ABSTRACT 2 Claims, 3 Drawingfigures 5 LT/M/NG C/RCU/T CAPACITOR DISCHARGE TYPE IGNITION SYSTEM FORINTERNAL COMBUSTION ENGINES BACKGROUND OF THE INVENTION 1. Field of theInvention The invention relates to improvements in a capacitor dischargetype ignition system for internal combustion engines.

2. Description of the Prior Art The prior art capacitor discharge typeignition systems for internal combustion engines are of the constructionshown in FIG. 1, comprising a discharge capacitor 1, an ignition coil 2having a primary winding 3 and a secondary winding 4, a siliconcontrolled rectifying element 5 (hereinafter referred to as SCR) servingas a switching element, a timing circuit 5" generating trigger pulses inthe ignition time of an engine, a diode 6 for the AC spark operation, achoke coil 7 for maintaining the spark, a distributor 8 and a spark plug9.

In operation, when the SCR 5 is triggered upon the impression of anignition signal to the gate, the discharge capacitor 1 is dischargedthrough the rimary winding 3 of ignition coil 2 to induce a high voltageacross the secondary winding, thereby causing a spark to pass across thegap of the spark plug 9. Thereafter, the polarity of the charge in thecapacitor 1 is reversed to the polarity opposite to that illustrated inthe figure by the free oscillation of the current flowing through theprimary winding 3, and then the capacitor 1 is re-discharged through thediode 6 and the choke coil 7 and returns to the original polarity,thereby reproducing a second spark in the spark plug 9. When the numberof turns of the primary winding 3 is decreased to reduce the inductanceof the primary winding 3, the discharge of the capacitor 1 is repeatedat a higher frequency, so that an extremely strong igniting action ofthe spark plug is attained even when the electrodes of the spark plug 9are contaminated. An increase of the frequency of discharge of thecapacitor 1, however, results in a decrease of the duration of anindividual spark across the gap between the electrodes of the spark plug9, which gives rise to the problem of failure in firing. The duration ofthe spark may be increased to some extent by increasing the inductanceof the choke coil 7', but in doing so, a substantial portion of thedischarge voltage during the reverse discharge after the reversal of thepolarity of the charge in the capacitor 1 comes to be applied across theterminals of the choke coil 7, which is not magnetically coupled to thesecondary coil 4, thus eventually decreasing the spark duration insteadof advantageously increasing it.

SUMMARY OF THE INVENTION An object of the invention is to increase thespark duration, while at the same time obtaining a strong spark, evenwith contaminated spark plug electrodes by ensuring rapid rising of theoutput voltage on the secondary side of the ignition coil, by means ofsuch a construction that the total inductance of the primary side of theignition coil, namely the turn ratio of the ignition coil, is differentbetween the forward discharge and the subsequent reverse discharge ofthe discharge capacitor, which is attained by having the primary windingof the ignition coil connected at one end thereof to one terminal of thedischarge capacitor, providing the primary winding with an intermediatetap which is connected with the other terminal of the dischargecapacitor through a switching element such as an SCR and having theprimary winding of the ignition coil connected at the other end thereofto the aforesaid other terminal of the capacitor through a diodeinserted in the polarity opposite to that of the switching element or aseries circuit consisting of a diode of the above-mentioned polarity anda choke coil.

Another object of the invention is to prevent, by means of the aforesaidchoke coil, the reduction of the output voltage on the secondary side ofan ignition coil due to a short-circuit current caused by a voltageinduced across the additional portion of the primary winding during theforward discharge of the discharge capacitor.

According to the invention, as the primary winding of the ignition coilis connected at one end to one terminal of the discharge capacitor, isprovided with an intermediate tap which leads to the other terminal ofthe capacitor through a switching element and is connected, at the otherend thereof, to the aforesaid other terminal of the capacitor through adiode inserted in the polarity opposite to the switching element or aseries circuit consisting of a'diode of the above-mentioned polarity anda choke coil, during the conduction period of the switching element, thedischarge current from the capacitor flows through the intermediate tapand the switching element. During this period the turn-ratio of thesecondary to the primary windings is high with the excellent effect thatthe initiation of the discharge of the capacitor is extremely improved,and that the voltage induced across the secondary winding issufficiently high to produce an extremely strong spark, even withcontaminated electrodes of the spark plug. During the reverse dischargeperiod of the capacitor after the polarity reversal thereof, the entireturns of the pri mary winding are available for operation to give alower tumratio, the entire voltage across the capacitor is appliedacross the ends of the total primary winding, and the inductance isincreased by the added primary winding portion, thereby causing a largecurrent to flow through the secondary winding to ensure extending theduration of the spark in the spark plug with excellent results.

Further, through the co-operation of the choke coil with the additionalportion of the primary winding, it is possible to prevent the reductionof the output voltage on the secondary side during the forward dischargeof the capacitor.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a circuit diagram of aconventional capacitor discharge type ignition system.

FIG. 2 is a circuit diagram of a preferred embodiment of the capacitordischarge type ignition system according to the present invention.

FIG. 3 is a circuit diagram of another embodiment of the capacitordischarge type ignition system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will nowbe described in conjunction with the illustrated embodiments. Withreference to FIG. 2, where the reference numerals l to 5, 5', 6, 8 and 9designate the respective circuit elements identical with or equivalentto those shown in FIG. 1, the primary winding 3 is provided with anintermediate tap 3a, which is connected through as SCR 5 to one terminalof the discharge capacitor 1. Numeral 11 generally designates a DC-to-DCconverter including a stepup transformer 12 having a primary winding 13and a secondary winding 14, a switching transistor 15 and a sourcebattery 16.

In the operation of the ignition system of the foregoing constructionaccording to the present invention, a charge stored in the dischargecapacitor 1, upon triggering of the SCR 5, is discharged through theintermediate tap 3a of the primary winding 3 of the ignition coil 2 witha very short rise time. Because of this, a high voltage is inducedacross the secondary winding 4 to produce a spark between the electrodesof the spark plug 9. Subsequently, a charge which has been stored in thedischarge capacitor in the polarity opposite to that illustrated in thefigure is discharged through the diode 6 and the total turns of theprimary winding 3 to recover the previous polarity. As a result, highvoltage is again induced across the secondary winding 4, producing aspark between the electrodes of the spark plug 9.

In the operation as described above, when a spark is produced at thespark plug 9 upon the triggering of the SCR 5, the number of turns ofthe primary winding coupled to the secondary winding is the number ofturns of a portion between the terminals 3b and 3a within the totalprimary winding 3 between the terminals 3b and 30, being divided by theintermediate tap 3a, thereby resulting in a greater turn ratio of thesecondary winding 4 to the primary winding 3 than the case of theprimary number of turns of the total primary winding 3 between theterminals 3b and 3c, and the inductance of the portion between theterminals 3b and 3a is lower than that of the total primary windingbetween the terminals 3b and 30, so that the initiation of the dischargeof the capacitor 1 is extremely favorable, inducing a sufficiently highvoltage across the secondary winding to produce an extremely strongspark, even when the electrodes of the spark plug 9 are contarninated.

In the reverse discharge of the capacitor 1 after the polarity reversalthereof, a discharge current flows through the diode 6 and the totalturns of the primary winding 3 of the ignition coil 2 between theterminals 3c and 3b. In this case, the gap of the spark plug 9 is in anionized state due to the previous spark discharge, so that a spark maybe produced at a lower voltage across the electrodes of the spark plug9. Besides, in the reverse discharge, the total turns of the primarywinding 3 between the terminals 30 and 3b are coupled to the secondarywinding 4 with a lower turn ratio as compared to the turn ratio for theprevious forward discharge wherein only a portion of the primary winding3 between the terminals 3b and 3a is coupled to the secondary winding 4.This causes a larger current to flow through the secondary winding 4with a sacrifice in the induced voltage thereacross. Further, when thecapacitor is discharged after the reversal of the polarity, the totalvoltage across the capacitor 1 is effectively applied across the totalprimary winding 3 between the terminals 3b and 3c, which has anincreased inductance, so that the integral of a current flowing throughthe secondary winding 4 is increased to extend the duration of the sparkproduced at the spark plug 9.

It is to be understood that, in place of the SCR 5 in the foregoingembodiment, a triac, a gate-tum off thyristor, etc., may as well be usedfor the switching element.

By way of numerically exemplifying the system according to theinvention, with the ignition coil 2 having a primary winding 3 of 50turns for the portion between the terminals 3b and 3a and 150 turns forthe portion between the terminals 3a and 3c and the secondary winding 4of 3,750 turns, and with a spark plug having gap of 0.8 millimeter, itis possible to extend the length of the spark duration to 250microseconds, which is extremely long as compared to 100 microsecondswhen employing the conventional system.

Another embodiment of the invention will now be described with referenceto FIG. 3. It comprises a discharge capacitor 1, an ignition coil- 2with a primary winding 3 and secondary winding 4, an SCR 5, a diode 6, achoke coil 7, a distributor 8 and a spark plug 9. The primary winding 3is provided with an intermediate tap 3a connected to one terminal of thedischarge capacitor 1 through the SCR 5. Numeral l1 designates aDC-to-DC converter, and numeral 16 designates a battery.

With respect to its operation, when the ignition time of the engine isreached an ignition signal is fed to the gate of the SCR 5 to triggerthe SCR 5, whereupon a charge stored in the discharge capacitor 1 by theDC-to-DC converter begins to be discharged through the intermediate tap3a of the primary winding 3 of the ignition coil 2 and the SCR 5. As aresult, a high voltage is induced across the secondary winding 4 toproduce a spark across the gap of the spark plug 9. The operation so faris the same as the conventional capacitor discharge type ignitionsystem, maintaining the feature of producing an output voltage having ashort rise time and of producing a strong spark even with contaminatedelectrodes of the spark plug 9. In the subsequent period a maximumreverse voltage is reached across the discharge capacitor 1 due to thefree oscillation caused in the ignition coil 2, whereupon a reversedischarge commences through the choke coil 7, the diode 6 and the totalprimary winding 3 between the terminals 30 and 3b, again inducing a highvoltage across the secondary winding 4 of the ignition coil 4 to producea spark across the gap of the spark plug 9 by way of the distributor 8.At this time, the gap of the spark plug 9 is ionized due to the previousignition, so that the spark may be initiated by a fairly low voltage.When the discharge capacitor 1 recovers the previous forward polarityand the voltage rises to a maximum, the SCR 5 is already tumed off, sono further current flows.

It is evident from the comparison of the foregoing operation with theoperation of the conventional system that the reverse discharge of thedischarge capacitor 1 after the polarity reversal thereof, which is madethrough choke coil 7, diode 6 and the primary winding 3 in theconventional system, is also made through choke coil 7, diode 6 and thetotal primary winding 3 between the terminals 3c and 3b according to theinvention. Due to the fact that, the discharge capacitor 1 of thereverse polarity is discharged through a path such asmentioned above thefollowing two effects may be attained simultaneously. The first effectis that the period of the free oscillation caused by the charge storedin the discharge capacitor 1, the choke coil 7 and the total primarywinding between the terminals 3b and 30 can be extended. Particularly,the co-operation of the primary winding portion between the terminals 3aand 3b and the choke coil 7 makes it possible to control the period offree oscillation over a fairly wide range. The second effect is that theturn ratio of the secondary to the primary is reduced for the reversedischarge, since the total turns of the primary winding between theterminals 3b and 3c are coupled to the secondary winding of the ignitioncoil 2. Thus, in the discharge process of the stored charge of thereverse polarity, a much larger current (the integral value) can be madeto flow through the spark plug 9 than the case that only the primarywinding portion between the terminals 3a and 3b is coupled. Besides, thevoltage across the total primary winding between the terminals 3b and 3cis sufficiently high as compared with the voltage across the choke coil7, so that there is no possibility that the output voltage on thesecondary side of the ignition coil is too low to produce a sparkthrough the spark plug 9 as in the case of employing a choke coil 7having an extremely high inductance. By virtue of the foregoing twoeffects, it is possible to attain a sufficiently long duration of sparkacross the gap of the spark plug 9 for the reverse discharge of thedischarge capacitor 1.

Further, the co-operation of the portion of the primary winding betweenthe terminals 3a and 3c of the ignition coil 2 and the choke coil 7 hasthe following great advantage.

It is required that, during the discharge of the charge having thereverse polarity in the discharge capacitor 1, voltages of the samepolarity with the positive pole on the sides directed to the diode 6 arerespectively induced across the portion between the terminals 30 and 3band the portion between the terminals 3c and 3a of the primary winding 3of the ignition coil 2. While, during the forward discharge of thedischarge capacitor 1, a voltage is induced across the primary windingportion between the terminals 3a and 30 with the positive pole on theanode side of the SCR 5 which is connected with the terminal 3a. Sincethe SCR 5 is conductive in this case, a short-circuit current due to thevoltage induced across the primary winding portion between the terminals3a and 30 tends to flow through the SCR 5. However, as the choke coil 7is connected in series with the primary winding portion between theterminals 3a and 3c according to the invention, the shortcircuit currentis prevented by the inductance of the choke coil 7. Consequently, in theforward discharge process, the

reduction of the output voltage on the secondary side by the influenceof the primary winding portion between the terminals 3a and 30 may beprevented. By way of a numerical example, with the primary winding 3 of50 turns for the portion between the terminals 3a and 3b and turns forthe portion between the terminals 3a and 3c, the secondary winding 4 of3,750 turns and the choke coil 7 of about 2 mI-l, the output voltage onthe secondary side could be made 26 KV as compared with 22 KV in thecase without the choke coil 7.

I claim:

1. A capacitor discharge type ignition system for combustion engines,comprising:

means including a capacitor for storing and discharging ignition energyalternately in first and second directions,

an ignition coil having a primary winding with first and second serialportions and having an output winding with a greater number of turnsthan said primary winding and being magnetically coupled to both of saidportions,

means including unidirectional switching means coupling said firstprimary winding portion across said capacitor,

means for turning said switching means alternatively on and off to causefirst direction current from said capacitor to pass, when said switchingmeans is on, through said first portion only of said primary winding tocause a rapid rise in the voltage across said output winding, and

means, including a unidirectional device poled oppositely to saidunidirectional switching means and coupling said second portion of theprimary winding to said condenser so that all of said primary winding iselectrically connected across said capacitor when said unidirectionalswitching means is turned off as aforesaid to cause second directioncapacitor current to flow through said primary winding, for decreasingthe step-up ratio of the primary winding to said output winding andcausing a larger current to then flow through the output winding for aprolonged period of time than when the first direction current flowsthrough said primary winding.

2. A system as in claim 1 wherein first direction capacitor currenttends to circulate from said unidirectional switching

1. A capacitor discharge type ignition system for combustion engines,comprising: means including a capacitor for storing and dischargingignition energy alternately in first and second directions, an ignitioncoil having a primary winding with first and second serial portions andhaving an output winding with a greater number of turns than saidprimary winding and being magnetically coupled to both of said portions,means including unidirectional switching means coupling said firstprimary winding portion across said capacitor, means for turning saidswitching means alternatively on and off to cause first directioncurrent from said capacitor to pass, when said switching means is on,through said first portion only of said primary winding to cause a rapidrise in the voltage across said output winding, and means, including aunidirectional device poled oppositely to said unidirectional switchingmeans and coupling said second portion of the primary winding to saidcondenser so that all of said primary winding is electrically connectedacross said capacitor when said unidirectional switching means is turnedoff as aforesaid to cause second direction capacitor current to flowthrough said primary winding, for decreasing the step-up ratio of theprimary winding to said output winding and causing a larger current tothen flow through the output winding for a prolonged period of time thanwhen the first direction current flows through said primary winding. 2.A system as in claim 1 wherein first direction capacitor current tendsto circulate from said unidirectional switching means through saidunidirectional device and said second primary winding portion to inducein the output winding a voltage in opposition to the volTage inducedtherein by the first direction current in the first primary windingportion, the improvement of: means including a choke coil seriallyconnected to said unidirectional device for substantially eliminatingsaid opposition voltage.